Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop-on-demand ink jet or continuous ink jet.
The first technology, “drop-on-demand” ink jet printing, provides ink droplets that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). Many commonly practiced drop-on-demand technologies use thermal actuation to eject ink droplets from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink droplet. This form of ink jet is commonly termed “thermal ink jet (TIJ).” Other known drop-on-demand droplet ejection mechanisms include piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to van Lintel, on Jul. 6, 1993; thermo-mechanical actuators, such as those disclosed by Jarrold et al., U.S. Pat. No. 6,561,627, issued May 13, 2003; and electrostatic actuators, as described by Fujii et al., U.S. Pat. No. 6,474,784, issued Nov. 5, 2002.
The second technology, commonly referred to as “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets from a nozzle. The stream is perturbed in some fashion causing it to break up into substantially uniform sized drops at a nominally constant distance, the break-off length, from the nozzle. A charging electrode structure is positioned at the nominally constant break-off point so as to induce a data-dependent amount of electrical charge on the drop at the moment of break-off. The charged droplets are directed through a fixed electrostatic field region causing each droplet to deflect proportionately to its charge. The charge levels established at the break-off point thereby cause drops to travel to a specific location on a recording medium or to a gutter for collection and recirculation.
Continuous ink jet (CIJ) drop generators rely on the physics of an unconstrained fluid jet, first analyzed in two dimensions by F. R. S. (Lord) Rayleigh, “Instability of jets,” Proc. London Math. Soc. 10 (4), published in 1878. Lord Rayleigh's analysis showed that liquid under pressure, Pr, will stream out of a hole, the nozzle, forming a jet of diameter, Dj, moving at a velocity, vd. The jet diameter, Dj, is approximately equal to the effective nozzle diameter, Dn, and the jet velocity is proportional to the square root of the reservoir pressure, Pr. Rayleigh's analysis showed that the jet will naturally break up into drops of varying sizes based on surface waves that have wavelengths, λ, longer than πDj, i.e. λ≦πDj. Rayleigh's analysis also showed that particular surface wavelengths would become dominant if initiated at a large enough magnitude, thereby “synchronizing” the jet to produce mono-sized drops. Continuous ink jet (CIJ) drop generators employ some periodic physical process, a so-called “perturbation” or “stimulation”, which has the effect of establishing a particular, dominant surface wave on the jet. The surface wave grows causing the break-off of the jet into mono-sized drops synchronized to the frequency of the perturbation.
The drop stream that results from applying Rayleigh stimulation will be referred to herein as a stream of drops of predetermined volume as distinguished from the naturally occurring stream of drops of widely varying volume. While in prior art CIJ systems, the drops of interest for printing or patterned layer deposition were invariably of substantially unitary volume, it will be explained that for the present inventions, the stimulation signal may be manipulated to produce drops of predetermined substantial multiples of the unitary volume. Hence the phrase, “streams of drops of predetermined volumes” is inclusive of drop streams that are broken up into drops all having nominally one size or streams broken up into drops of selected (predetermined) different volumes.
In a CIJ system, some drops, usually termed “satellites” much smaller in volume than the predetermined unit volume, may be formed as the stream necks down into a fine ligament of fluid. Such satellites may not be totally predictable or may not always merge with another drop in a predictable fashion, thereby slightly altering the volume of drops intended for printing or patterning. The presence of small, unpredictable satellite drops is, however, inconsequential to the present inventions and is not considered to obviate the fact that the drop sizes have been predetermined by the synchronizing energy signals used in the present inventions. Thus the phrase “predetermined volume” as used to describe the present inventions should be understood to comprehend that some small variation in drop volume about a planned target value may occur due to unpredictable satellite drop formation.
Commercially practiced CIJ printheads use a piezoelectric device, acoustically coupled to the printhead, to initiate a dominant surface wave on the jet. The coupled piezoelectric device superimposes periodic pressure variations on the base reservoir pressure, causing velocity or flow perturbations that in turn launch synchronizing surface waves. A pioneering disclosure of a piezoelectrically-stimulated CIJ apparatus was made by R. Sweet in U.S. Pat. No. 3,596,275, issued Jul. 27, 1971, Sweet '275 hereinafter. The CIJ apparatus disclosed by Sweet '275 consisted of a single jet, i.e. a single drop generation liquid chamber and a single nozzle structure.
Sweet '275 disclosed several approaches to providing the needed periodic perturbation to the jet to synchronize drop break-off to the perturbation frequency. Sweet '275 discloses a magnetostrictive material affixed to a capillary nozzle enclosed by an electrical coil that is electrically driven at the desired drop generation frequency, vibrating the nozzle, thereby introducing a dominant surface wave perturbation to the jet via the jet velocity. Sweet '275 also discloses a thin ring-electrode positioned to surround but not touch the unbroken fluid jet, just downstream of the nozzle. If the jetted fluid is conductive, and a periodic electric field is applied between the fluid filament and the ring-electrode, the fluid jet may be caused to expand periodically, thereby directly introducing a surface wave perturbation that can synchronize the jet break-off. This CIJ technique is commonly called electrohydrodynamic (EHD) stimulation.
Sweet '275 further disclosed several techniques for applying a synchronizing perturbation by superimposing a pressure variation on the base liquid reservoir pressure that forms the jet. Sweet '275 disclosed a pressurized fluid chamber, the drop generator chamber, having a wall that can be vibrated mechanically at the desired stimulation frequency. Mechanical vibration means disclosed included use of magnetostrictive or piezoelectric transducer drivers or an electromagnetic moving coil. Such mechanical vibration methods are often termed “acoustic stimulation” in the CIJ literature.
The several CIJ stimulation approaches disclosed by Sweet '275 may all be practical in the context of a single jet system However, the selection of a practical stimulation mechanism for a CIJ system having many jets is far more complex. A pioneering disclosure of a multi-jet CIJ printhead has been made by Sweet et al. in U.S. Pat. No. 3,373,437, issued Mar. 12, 1968, Sweet '437 hereinafter. Sweet '437 discloses a CIJ printhead having a common drop generator chamber that communicates with a row (an array) of drop emitting nozzles. A rear wall of the common drop generator chamber is vibrated by means of a magnetostrictive device, thereby modulating the chamber pressure and causing a jet velocity perturbation on every jet of the array of jets.
Since the pioneering CIJ disclosures of Sweet '275 and Sweet '437, most disclosed multi-jet CIJ printheads have employed some variation of the jet break-off perturbation means described therein. For example, U.S. Pat. No. 3,560,641 issued Feb. 2, 1971 to Taylor et al. discloses a CIJ printing apparatus having multiple, multi-jet arrays wherein the drop break-off stimulation is introduced by means of a vibration device affixed to a high pressure ink supply line that supplies the multiple CIJ printheads. U.S. Pat. No. 3,739,393 issued Jun. 12, 1973 to Lyon et al. discloses a multi-jet CIJ array wherein the multiple nozzles are formed as orifices in a single thin nozzle plate and the drop break-off perturbation is provided by vibrating the nozzle plate, an approach akin to the single nozzle vibrator disclosed by Sweet '275. U.S. Pat. No. 3,877,036 issued Apr. 8, 1975 to Loeffler et al. discloses a multi-jet CIJ printhead wherein a piezoelectric transducer is bonded to an internal wall of a common drop generator chamber, a combination of the stimulation concepts disclosed by Sweet '437 and '275
Unfortunately, all of the stimulation methods employing a vibration of some component of the printhead structure or a modulation of the common supply pressure result in some amount of non-uniformity of the magnitude of the perturbation applied to each individual jet of a multi-jet CIJ array. Non-uniform stimulation leads to a variability in the break-off length and timing among the jets of the array. This variability in break-off characteristics, in turn, leads to an inability to position a common drop charging assembly or to use a data timing scheme that can serve all of the jets of the array.
In addition to addressing problems of break-off time control among jets of an array, continuous drop emission systems that generate drops of different predetermined volume based on liquid pattern data need a means of stimulating each individual jet in an independent fashion in response to the liquid pattern data. Consequently, in recent years an effort has been made to develop practical “stimulation per jet” apparatus capable of applying individual stimulation signals to individual jets.
The electrohydrodynamic (EHD) jet stimulation concept disclosed by Sweet '275 operates on the emitted liquid jet filament directly, causing minimal acoustic excitation of the printhead structure itself, thereby avoiding the above noted confounding contributions of printhead and mounting structure resonances. U.S. Pat. No. 4,220,958 issued Sep. 2, 1980 to Crowley discloses a CIJ printer wherein the perturbation is accomplished by an EHD exciter composed of pump electrodes of a length equal to about one-half the droplet spacing. The multiple pump electrodes are spaced at intervals of multiples of about one-half the droplet spacing or wavelength downstream from the nozzles. This arrangement greatly reduces the voltage needed to achieve drop break-off over the configuration disclosed by Sweet '275.
While EHD stimulation has been pursued as an alternative to acoustic stimulation, it has not been applied commercially because of the difficulty in fabricating printhead structures having the very close jet-to-electrode spacing and alignment required and, then, operating reliably without electrostatic breakdown occurring. Also, due to the relatively long range of electric field effects, EHD is not amenable to providing individual stimulation signals to individual jets in an array of closely spaced jets.
An alternate jet perturbation concept that overcomes all of the drawbacks of acoustic or EHD stimulation was disclosed for a single jet CIJ system in U.S. Pat. No. 3,878,519 issued Apr. 15, 1975 to J. Eaton (Eaton hereinafter). Eaton discloses the thermal stimulation of a jet fluid filament by means of localized light energy or by means of a resistive heater located at the nozzle, the point of formation of the fluid jet. Eaton explains that the fluid properties, especially the surface tension, of a heated portion of a jet may be sufficiently changed with respect to an unheated portion to cause a localized change in the diameter of the jet, thereby launching a dominant surface wave if applied at an appropriate frequency. U.S. Pat. No. 4,638,328 issued Jan. 20, 1987 to Drake, et al. (Drake hereinafter) discloses a thermally-stimulated multi-jet CIJ drop generator fabricated in an analogous fashion to a thermal ink jet device. That is, Drake discloses the operation of a traditional thermal ink jet (TIJ) edgeshooter or roofshooter device in CIJ mode by supplying high pressure ink and applying energy pulses to the heaters sufficient to cause synchronized break-off but not so as to generate vapor bubbles.
Also recently, microelectromechanical systems (MEMS), have been disclosed that utilize electromechanical and thermomechanical transducers to generate mechanical energy for performing work. For example, thin film piezoelectric, ferroelectric or electrostrictive materials such as lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), or lead magnesium niobate titanate (PMNT) may be deposited by sputtering or sol gel techniques to serve as a layer that will expand or contract in response to an applied electric field. See, for example Shimada, et al. in U.S. Pat. No. 6,387,225, issued May 14, 2002; Sumi, et al., in U.S. Pat. No. 6,511,161, issued Jan. 28, 2003; and Miyashita, et al., in U.S. Pat. No. 6,543,107, issued Apr. 8, 2003. Thermomechanical devices utilizing electroresistive materials that have large coefficients of thermal expansion, such as titanium aluminide, have been disclosed as thermal actuators constructed on semiconductor substrates. See, for example, Jarrold et al., U.S. Pat. No. 6,561,627, issued May 13, 2003. Therefore electromechanical devices may also be configured and fabricated using microelectronic processes to provide stimulation energy on a jet-by-jet basis.
U.S. Pat. No. 6,505,921 issued to Chwalek, et al. on Jan. 14, 2003, discloses a method and apparatus whereby a plurality of thermally deflected liquid streams is caused to break up into drops of large and small volumes, hence, large and small cross-sectional areas (Chwalek '921 hereinafter). Thermal deflection is used to cause smaller drops to be directed out of the plane of the plurality of streams of drops while large drops are allowed to fly along nominal “straight” pathways. In addition, a uniform gas flow is imposed in a direction having velocity components perpendicular and across the array of streams of drops of cross-sectional areas. The perpendicular gas flow velocity components apply more force per mass to drops having smaller cross-sections than to drops having larger cross-sections, resulting in an amplification of the deflection acceleration of the small drops.
U.S. Pat. No. 6,588,888 entitled “Continuous ink-jet printing method and apparatus,” issued to Jeanmaire, et al. (Jeanmaire '888, hereinafter) and U.S. Pat. No. 6,575,566 entitled “Continuous inkjet printhead with selectable printing volumes of ink,” issued to Jeanmaire, et al. (Jeanmaire '566 hereinafter) disclose continuous ink jet printing apparatus including a droplet forming mechanism operable in a first state to form droplets having a first volume traveling along a path and in a second state to form droplets having a plurality of other volumes, larger than the first, traveling along the same path. A droplet deflector system applies force to the droplets traveling along the path. The force is applied in a direction such that the droplets having the first volume diverge from the path while the larger droplets having the plurality of other volumes remain traveling substantially along the path or diverge slightly and begin traveling along a gutter path to be collected before reaching a print medium. The droplets having the first volume, print drops, are allowed to strike a receiving print medium whereas the larger droplets having the plurality of other volumes are “non-print” drops and are recycled or disposed of through an ink removal channel formed in the gutter or drop catcher.
In preferred embodiments, the means for variable drop deflection comprises air or other gas flow. The gas flow affects the trajectories of small drops more than it affects the trajectories of large drops. Generally, such types of printing apparatus that cause drops of different sizes to follow different trajectories, can be operated in at least one of two modes, a small drop print mode, as disclosed in Jeanmaire '888 or Jeanmaire '566, and a large drop print mode, as disclosed also in Jeanmaire '566 or in U.S. Pat. No. 6,554,410 entitled “Printhead having gas flow ink droplet separation and method of diverging ink droplets,” issued to Jeanmaire, et al. (Jeanmaire '410 hereinafter) depending on whether the large or small drops are the printed drops. The present invention described hereinbelow are methods and apparatus for implementing either large drop or small drop printing modes.
The combination of individual jet stimulation and aerodynamic deflection of differently sized drops yields a continuous liquid drop emitter system that eliminates the difficulties of previous CIJ embodiments that rely on some form of drop charging and electrostatic deflection to form the desired liquid pattern. Instead, the liquid pattern is formed by the pattern of drop volumes created through the application of input liquid pattern dependent drop forming pulse sequences to each jet, and by the subsequent deflection and capture of non-print drops. An additional benefit is that the drops generated are nominally uncharged and therefore do not set up electrostatic interaction forces amongst themselves as they traverse to the receiving medium or capture gutter.
However this configuration of liquid pattern deposition has some remaining difficulties when high speed, high pattern quality printing is undertaken. High speed and high quality liquid pattern formation requires that closely spaced drops of relatively small volumes are directed to the receiving medium. As the pattern of drops traverse from the printhead to the receiving medium, through a gas flow deflection zone, the drops alter the gas flow around neighboring drops in a pattern-dependent fashion. The altered gas flow, in turn, causes the printing drops to have altered, pattern-dependent trajectories and arrival positions at the receiving medium. In other words, the close spacing of print drops as they traverse to the receiving medium leads to aerodynamic interactions and subsequent drop placement errors. These errors have the effect of spreading an intended printed liquid pattern in an outward direction and so are termed “splay” errors herein.
Therefore to gain full advantage of the simplification in continuous liquid drop emitter printhead structure offered by individual jet stimulation and aerodynamic drop deflection, practical and efficient methods of reducing aerodynamic interaction error are needed.